The Filter

The frequencies of interest are under 600 Hz. I need to put a filter just after the microphone to attenuate the unwanted frequencies. I would like this filter to take up as little PCB real estate as possible, make it very small and simple.

I would like a second order low pass filter. A passive RC network is the simplest, but is first order. A second order RC network contains at least four components, and also attenuates the passband.

An active filter has a second order implementation, but is much more complex. I would need to supply power to the active devices, the foot print would be much larger, and there would be more parts to fail. I would rather not use any of these implementations.

The simplest second order filter possible is likely an RLC filter.  To make a low pass, second order configuration, the inductor is in series with the signal, the capacitor is in parallel with the signal. The inductor is a short to small frequencies and is an open to large frequencies. The capacitor is an open to small frequencies, and a short to large frequencies.

The input is a 1 kilo Ohm resistance microphone, CMB-6544PF. The output is an infinite impedance amplifier.

This is the circuit. The one kOhm resistor is part of the signal source. This creates a difficulty.

Converting L and C to their equivalent S domain impedances.

Equating terms.

and

So, accordingly.

There lies the problem. Because the output impedance of the microphone is so large, the resulting inductance is very large. To have an inductance that has an economical value, the resistance would need to be very small. An inductance of this size is likely to be very large, and have a lot more problems associated with it.

After some thought, a first order RC filter seems the best option.

A QUCS simulation confirms that while both components are more physically realizable, the point where the attenuation reaches 10 % is at 5 kHz.

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Electrical Engineer, Minneapolis, Minnesota